1. Field of the Invention
The present invention relates to a process for producing a glass master and a stamper for production of an optical disk, particularly of a high-density recording optical disk. This process is especially effective for producing a domain wall displacement type of magneto-optical medium.
2. Related Background Art
A conventional process for producing a stamper for optical disks is explained by reference to FIGS. 1A to 1G.
In this process, as shown in FIG. 1A, positive type photoresist 2 is applied by spin-coating onto master glass 1 which has been polished and has been spin-coated as necessary with a primer like a silane-coupling agent. The photoresist-coated master is prebaked in a clean oven or a like apparatus. Then as shown in FIG. 1B, predetermined regions of glass master substrate is exposed to light by a light beam modulated in accordance with recording signals by means of a light exposure system (cutting machine) employing an argon ion laser or the like as the light source. In the drawings, the numeral 3 indicates a light beam of the cutting machine, the numeral 4 indicates an exposed region, and the numeral 5 indicates an unexposed region. Generally, the photoresists are classified into negative type photoresists for utilizing the exposed region and positive type photoresists for utilizing unexposed region: the two types of the photoresists are selectively used to meet the objects of use. In the process explained, in the next step, as shown in FIG. 1C, the light-exposed glass master substrate is washed with a developing solution composed of an inorganic alkali and ultrapure diluting water to remove the light-exposed region, washed with pure water, dried by spin-drying, and post-baked to obtain an optical disk master. The photoresist-removed region forms a groove 9, and the remaining photoresist region forms a land 8.
In the step of FIG. 1D, the surface of the optical disk master is coated with electroconductive film 6 such as a nickel film by sputtering. In the step of FIG. 1E, nickel is electroformed on the electroconductive nickel film. The numeral 15 indicates a resulting electroformed nickel layer. The back face of the electroformed nickel layer is polished. Then in the step shown in FIG. 1F, a metal stamper is peeled from the optical disk master. Thereby, a stamper 7 is completed as shown in FIG. 1G.
In recent years, a process is disclosed which produces an optical disk substrate stamper by anisotropic etching to obtain a deep-groove stamper for forming grooves having a nearly rectangular cross-section. Japanese Patent Application Laid-Open No. 7-161080, for example, discloses a process for producing a stamper for optical disk substrates by reactive ion etching (RIE). This process for production and working of the stamper for a land-groove recording substrate by the reactive ion etching is explained by reference to FIGS. 2A to 2I.
In this process, as shown in FIG. 2A, positive type photoresist 2 is applied by spin-coating onto synthetic quartz master 10 which has been polished, washed sufficiently, and spin-coated with a primer as necessary. The photoresist-coated master is prebaked in a clean oven or a like apparatus. Then as shown in FIG. 2B, a predetermined region of the master is exposed to light by a light beam 3 modulated in correspondence with recording signals by a light exposure system (cutting machine) employing an argon ion laser or the like as the light source. In the next step, as shown in FIG. 2C, the light-exposed master is developed by spinning with a developing solution containing an inorganic alkali to remove exposed region 4 of the photoresist. The master is washed by pure water shower, dried by spinning. The unexposed portion 5 is post-baked in a clean oven.
In the next step as shown in FIG. 2D, the master is placed in a chamber of a reactive ion etching apparatus. The chamber is evacuated to a vacuum degree of 1×10−4 Pa. Therein CHF3 or a like gas in introduced, and reactive ion etching is conducted to obtain glass master 13.
Then as shown in FIG. 2E, the remaining resist is peeled off from glass master 13 by immersing the glass master into a peeling solution composed of concentrated sulfuric acid and aqueous hydrogen peroxide to obtain an optical disk master. In the drawing, the numeral 8 indicates a land, and the numeral 9 indicates a groove. Next, as shown in FIG. 7F, the glass master 13 is washed, and on the surface thereof an electroconductive Ni film 6 is formed by sputtering to impart electroconductivity. Further, as shown in FIG. 2G, a Ni layer is electroformed thereon. The numeral 15 indicates the electroformed Ni layer. The electroformed layer surface is polished, and the electroformed Ni layer 15 is peeled from glass master 13 as shown in FIG. 2H. Through the above steps, stamper 7 is completed (FIG. 2I).
Other processes are disclosed for producing and working a stamper for a land-groove recording substrate, in which a thin film such as an SiO2 thin film is formed on a master glass made of conventional glass produced from soda lime, not expensive quartz glass, as the glass master by reactive ion etching (RIE). Japanese Patent Application Laid-Open No. 11-336748, for example, discloses a process for producing and working a stamper for optical disk substrate, in which plural thin films are formed in lamination on a master glass, a photoresist is applied thereon, the photoresist is exposed to light and is developed, and reactive ion etching is conducted.
Explanation is made on a process for producing and working a stamper by a reactive ion etching of an inorganic oxide thin film such as SiO2 film formed on a master glass, by reference to FIGS. 3A to 3L.
Master glass 1 is polished and washed well. On the master glass 1, Al2O3 film 21 is formed as a first thin film layer as shown in FIG. 3A.
Thereon, SiO2 film 22 is formed in a thickness of about 140 nm as a second thin film layer by sputtering as shown in FIG. 3B.
The surface of the SiO2 film 22, which may be spin-coated with a primer if necessary, is spin-coated with positive photoresist 2 as shown in FIG. 3C. Then a prescribed region of the photoresist is exposed to light beam 3 modulated in accordance with recording signals by a light exposure apparatus (cutting machine) employing an argon ion laser or the like as the light source as shown in FIG. 3D. The photoresist is developed with the aforementioned inorganic alkali developing solution by spinning to remove exposed portion 4, as shown in FIG. 3E. The developed photoresist is washed with pure water, dried by spinning, and post-baked in a clean oven, as the post-treatment. The master is placed in a chamber of a reactive ion etching apparatus. The chamber is evacuated to a vacuum degree of 1×10−4 Pa; therein CHF3 or a like gas in introduced; and reactive ion etching 14 is conducted as shown in FIG. 3F. The etching is conducted to reach a prescribed groove depth (corresponding to the thickness of the second thin SiO2 film 22) to obtain glass master 13. From the glass master, the remaining resist is peeled by oxygen plasma ashing as shown in FIG. 3G. The glass master 13 is immersed into a developing stock solution to remove first thin film 21 of Al2O3 in the exposed region by wet etching to obtain optical disk master 13 as shown in FIG. 3H. The portion indicated by the numeral 8 becomes a land portion, and the portion indicated by the numeral 9 becomes a groove. On the surface of the glass master 13, after washing, electroconductive Ni film 6 is formed by sputtering for electroconductivity as shown in FIG. 3I. Thereon a nickel layer is electroformed as shown in FIG. 3J. The numeral 15 indicates a electroformed Ni layer. Then electroformed Ni layer 15 is peeled off from glass maser 13 as shown in FIG. 3K.
Stamper 7 is completed (FIG. 3L) through the steps shown above. In some application fields, the stripped metal stamper can be utilized, after insulation-film treatment of the surface, as a stamper family of mother-stamper/sun-stamper (not shown in the drawing). Thereafter, the stamper is worked by press-punching or a like working into a desired shape to complete a metal stamper. By use of this metal stamper, optical disk substrates having signal-recording hollows are replicated by injection molding, a 2P process, or the like molding method. On the optical disk substrate, a reflection film of a metal like aluminum or a magnetic film is formed to obtain an optical disk.
The stamper prepared by the above process has disadvantage that the faces of land portion 8, especially unexposed portion 5, and faces of the inclined side wall of the hollows become rough as shown in FIG. 4. This surface roughening can occur in the cutting step of FIG. 1B, in the sputtering step of FIG. 1D, or in the step of resist removal of the stamper. The surface roughening can be caused in the sputtering step by particle size and uniformity of the sputtering film, in the step of the resist removal by the resist-removing chemical. However, the cutting operation is the major cause of the surface roughening.
The light beam modulated by recording information for light exposure has a Gauss distribution of the intensity. Therefore, the light-exposed region is not precisely coincide with the intended region. This leakage of the light, and reverse face reflection can cause light exposure outside the intended region. The unintended light-exposed region can also be developed to cause fine surface roughness.
In a metal stamper prepared by reactive ion etching also, the side wall surfaces of the grooves are confirmed to be rough considerably although the surface roughness is nearly the same level between the land portions and groove portions. This can be caused by fine residues of the photoresist remaining on the interface between the photoresist and master glass after the light beam exposure and development.
This roughening problem becomes more serious at the narrower track pitch and the larger projection formed, and is important for increasing the density of the optical disk.
Various attempt are made for increasing the recording density of the optical disk. Japanese Patent Application Laid-Open No. 6-290496 discloses increase of the recording density of magneto-optical recording by domain wall displacement detection system. The domain wall displacement system achieves reproduction resolution exceeding the limit by light spot diameter in the line (track) direction by utilizing magnetic wall displacement caused by reading-out light spot. In magneto-optical record reproduction with such a substrate having the roughness of the side wall portion, the reflected light quantity varies by scattering of the reproduction light spot by the side wall roughness to increase substrate noise in the reproduced information signals to lower the S/N ratio of the signals. In particular, when the domain wall displacement detection type of magneto-optical recording medium is combined with such a deep-grooved substrate, the wrinkling roughness of tens of nanometers formed at the shoulder of the land portions prevents smooth displacement of the domain walls, in addition to the drop of S/N ratio of the signal, disadvantageously.
To solve the above problem, a known method is a baking treatment to improve the surface smoothness. In this method, the photoresist is exposed to a light beam, and a pattern is formed by development in a conventional production process, and a hard baking treatment is conducted in which the photoresist layer is heat-treated near the melting point thereof to improve the surface smoothness of the grooves.
In this improved production process, most of the side wall portion becomes gently sloping, and the roughness is decreased. However, the roughness is observed to be still remaining at the interface between the flat portion of the groove and the side wall by scanning electron microscopy.